This invention relates to welding of plastic sheet material, and in particular it relates to continuous welding together of sheets of the type which adhere by melting and re-hardening together of the sheet material itself.
It is known to form a strip of packaging material into a tube by passing the strip material over an external former. Such a former is shown in U.S. Pat. No. 2,899,875, issued Aug. 18, 1959. In the tube thus formed, the opposite edges of the sheet overlap and a means must be provided for sealing these edges together to complete the formation of the tube.
In one previous arrangement for sealing these overlapping sheet edges, the strip material is moved continuously and a heating element is applied to the overlapped longitudinal edges of the strip material as the material passes over a backing plate which is stationary relative to the moving film of material. This type of sealing has been found to be highly satisfactory in connection with the sealing of materials wherein an adhesive coating is applied to the facing surfaces of the overlapping edges. However, difficulties have been experienced when attempting to seal together by welding on a continuous basis materials such as polyethylene which do not utilize an adhesive coating but wherein the sheet itself is melted and re-hardened to form a weld connecting the sheets together.
These difficulties are derived from the fact that such self-welding materials such as polyethylene have a melting point temperature very close to the freezing point temperature, i.e. there is very little if any "soft" range. Consequently a small variation in temperature has an appreciable effect on the seal thus formed. Moreover, a certain length of time is required for the molten polyethylene to re-harden and thus complete the bond of the overlapping edges. Thus, in forming a weld of material such as polyethylene or the like, it has previously been necessary to stop the longitudinal movement of the sheet relative to the heating element and backing plate to allow the sheet material to freeze, i.e. harden, as a result of which the welding procedure is no longer truly continuous. If the sheet material is not stopped in this manner, owing to the said certain length of time required for the molten polyethylene to re-harden and thus complete the weld, some of the melted sheet material adjacent the surface of the backing plate, not having sufficient time to re-harden before the sheet material is carried away from the backing plates, will sometimes stick to this backing plate. The effect of this sticking is to separate the sheets at the seal. Also the molten material which sticks to the backing plate comes from the seal, thus further disturbing the seal and causing holes or "holidays" in the seal. In addition, this build-up of material on the backing plate will reduce the efficiency of the backing plate itself as it will no longer be a smooth surface over which the sheet material can easily slide.
In U.S. Pat. No. 3,133,390, one solution to this problem is provided, although this solution is specifically with respect to an interrupted, non-continuous, reciprocating seal packaging machine and relates primarily to thin film on the order of 0.001" to 0.008" thickness. As disclosed in U.S. Pat. No. 3,133,390, the backing plate is made to travel with the film while the seal is being formed afterwhich the backing plate must be intermittently returned to its original position for a subsequent seal. This intermittent movement is not a disadvantage in a form, fill and seal packaging machine wherein the tubular length of material must be stopped at intervals anyway to fill and transversely seal the individual packages. However, it is a distinct disadvantage in a continuous tube sealing process such as a pipe coating process in which it is desired to move the tube continuously while of course continuously sealing the overlapping edges thereof. In practice, a coated pipe might normally be cut into sections 40' in length.
Thus, there exists a need for improvements which will permit continuous sealing of overlapping edges of a tube of continuous length wherein the seal is formed by melting and re-hardening the sheet material itself.